In order to provide for accurate tightening of screws, screw threads must be meticulously measured for uniformity. The "pitch" of a screw thread is the distance between a point on a screw thread and the corresponding point on the adjacent thread in a line parallel with the axis of the screw. Moreover, the "pitch diameter" of a screw is the diameter of a screw measured from a midpoint on the flank surface of a screw thread through the screw to an opposite midpoint on a flank of the screw thread of the screw, in a direction perpendicular to the axis of the screw. In other words, on a straight thread, the pitch diameter is the diameter of the cylinder whose surface passes through the thread profiles at such points as to make the widths of thread groove and thread ridge equal.
In addition to pitch diameter, the major diameter of a screw thread, which is the distance from the outside of one side of the screw thread to the opposite side, is conventionally measured by a standard micrometer.
The minor diameter, however, which is the diameter from the interior base of the screw thread to an opposite interior base of the screw thread, is conventionally measured by a ring gauge, which follows the surface of the thread.
Various methods have been provided for measuring screw threads by using these aforementioned three wires for measuring the pitch diameter of the screw threads. Screw threads are measured in accordance with United States Department of Commerce, National Bureau of Standards HANDBOOK H28, entitled "Screw-Thread Standards For Federal Services-Appendix For Methods of Wire Measurement of Pitch Diameter of 60 degree Threads". According to the federal standards, the measurement of the pitch diameter, that is the distance between the centers of two adjacent screw threads, the measurement must be accurate with no room for movement of the wires, which are placed upon the flank surfaces within the screw threads for the purpose of measuring the pitch and the pitch diameter.
It is further noted that while the aforementioned appendix applies to 60 degree threads, other angled threads are similarly measured.
In a typical orientation one elongated center wire is placed upon the flank surfaces within the tapered walls of the screw thread. Opposite to this center wire are two offset wires upon the flank surfaces in the corresponding tapered walls between the threads of the screw.
Typically small hardened steel longitudinally extending cylinder wires are placed in the thread groove, the two offset wires on one side of the screw and the one single center wire on the opposite side. In the prior art, various holders are described for holding the wires within a micrometer anvil so that one holder, such as that having the two offset wires, is advanced towards the opposite holder with the center wire.
As noted above, the wires should be placed such that the outside circumference of the cylindrical wire touches exactly the mid slope of the flank of the thread of a given pitch. In the prior art, in order to hold the wires exactly in the proper measuring position, two movable holding members, such as rubber bands or springs, are typically used to hold the wires still in place during the measurement of the screw threads. However, use of the "movable" holders, such as rubber bands or springs, is frowned upon in government standards as well as in standard engineering texts, such as the Machinery Handbook, which notes that the three measuring wires should be firmly held in place during measurement.
When standard micrometers are used, however, often the use of the three-wire method for measuring screw threads is an unwieldy manual process, in which the user must hold the anvil of the micrometer in one hand, as well as the holders for the three wires, while the other hand moves the handle to rotate the spindle of the micrometer and thereby move the double offset wires towards the single center wire.
Various devices have been patented to attempt to use the three-wire method for measuring screw threads. Among these are U.S. Pat. Nos. 2,431,021 of Bourdelais, 2,939,220 of Croshier, 2,822,624 of Klink, 2,692,438 of Schneider, 2,906,029 also of Croshier, 2,962,817 of Barney, 4,480,388 of O'Brien, 4,821,422 of Porter and 4,939,845 also of Porter.
These patents generally disclose wire holding attachments for the three-wire method of measuring pitch diameters of screw threads. However, some of the known devices have significant disadvantages, such as the fact that the devices cannot hold the three wires firmly in place, within the screw threads, because of the use of springs which, as the springs are compressed smaller, results in the likelihood that there is increasing pressure upon the three wires, which may result in the possibility of the dislodging of the wires.
Such use of springs is disclosed in the Croshier '220 and '029 patents. In Croshier '220, two off-set wires, which are held in place against the screw threads, are movable, which can cause inaccuracies of the measurements. Likewise, in Croshier '029, the wires are held apart by a hub, so that the wire pins float, and are not locked in a solid position against the flank surfaces within the screw threads.
Furthermore, in Croshier '220, the holes holding the wires are larger than the diameter of the cylindrical wires, which may cause the wires to shift while conforming to the flank surfaces of the screw threads. The shifting may result in inaccurate measurements and is therefore contraindicated according to the aforementioned Federal standards and engineering handbooks.
Moreover, O'Brien '388 requires a flexible holder, such as a rubber band, which is not permitted under the federal Handbook H28.
The remaining patents have complicated moving parts and springs, which do not provide for accurate measurement of screw threads. Another problem with the prior art devices, such as O'Brien '388, is that they do not have a positive means for the wires to adjust to the lead angle of the threads being measured.
In an unpatented prior art publication, there is shown a three wire measuring system in the catalog of the Mahr Gage Co., Inc. of 274 Lafayette St., New York, N.Y., which defines a calibration device which includes a pair of holder ear pieces for holding the respective center wire and two off-set wires against the screw threads at opposite sides of the screw threads being measured. In the Mahr system, the two sets of wires move in place as the spindle of a micrometer is advanced towards the other wire holder with the single center wire.
The disadvantage of the Mahr device is that the wires are advanced towards each other in a horizontal orientation, thereby resulting in the possibility of inaccuracies, because of the effect of gravity upon the horizontally oriented holder ear pieces for the wires. In the Mahr the wires are solely held in place against gravity in a horizontal position by the wire holder ears. Moreover, in the Mahr device the wires are held in place in one location within the holder ear pieces, so that if the wire tends to wear out where it is being used to measure on a frequent basis, the wires cannot be moved in an axial direction so as to expose a further portion of the wire, which is not worn, out to the screw threads.
Moreover, a further disadvantage of the prior art is the use of a complicated ring gauge, which must travel the length of the screw threads, to measure the minor diameter of the screw threads.